The turbine wheels of automotive turbochargers are required to have high-temperature heat resistance since the turbine wheels are exposed to the high-temperature gas discharged from the engines. Alloys having excellent heat resistance, such as Ni-based alloys and Ti—Al alloys, have hence been used as the turbine wheels.
Ti—Al alloys are slightly inferior in oxidation resistance to Ni-based alloys such as Inconel (registered trademark) 713C. It is, however, known that the oxidation resistance is improved by adding Nb, Si, etc. to the Ti—Al alloys. In addition, since the amount of oxygenic components contained in actual automotive exhaust gases is small, the problem due to oxidation is being overcome.
Meanwhile, the temperature of the exhaust gases tends to rise as a result of the trend toward improvements in fuel efficiency and combustion efficiency, and improvements in strength property at high temperatures exceeding 900° C. are becoming an important subject.
In order to solve this problem, various proposals have hitherto been made.
For example, Patent Document 1 discloses a Ti—Al-based alloy which includes 38 to 45 at.% of Al and 3 to 10 at.% of Mn, with the balance being Ti and unavoidable impurities.
The document describes that the Ti—Al-based alloy can be made to combine machinability and high-temperature strength by suitably controlling the lamellar structure and the β phase within the Ti—Al-based alloy.
Patent Document 2 discloses a Ti—Al-based alloy which includes 38 to 48 at.% of Al and 4 to 10 at.% of Mn, with the balance being Ti and unavoidable impurities.
This document describes that the room-temperature ductility and, in particular, impact properties of the Ti—Al-based alloy are greatly improved when the alloy has a specific average grain diameter.
Patent Document 3 discloses a process for producing an alloy based on a Ti—Al-based intermetallic compound, the process including:
(1) subjecting a Ti—Al-based alloy containing 42 to 52 at.% of Al to grain fining by working the alloy at a strain rate of 1/sec or higher in an α-Ti single phase region with a temperature higher than 1,300° C.; and
(2) conducting a lamella formation treatment in which lamellae of TiAl and Ti3Al are yielded within the fine crystal grains obtained, thereby producing a fine lamellar grain structure.
This document indicates that a structure which is entirely configured of fine lamellar grains has an excellent property balance among ordinary-temperature ductility, high-temperature strength, and fracture toughness.
Furthermore, Patent Document 4 discloses a process for producing a Ti—Al intermetallic compound containing a lamellar structure, in which a heat treatment for increasing the lamellar layer spacing is performed at a temperature not higher than the solidus temperature.
This document describes that, by controlling the lamellar layer spacing, properties according to purposes (strength, hardness, heat resistance, impact resistance, etc.) can be controlled.
As described in Patent Documents 1 to 4, to control the structure of a Ti—Al-based alloy is effective in improving the mechanical properties of the Ti—Al-based alloy. However, there are limitations on the improvements in mechanical property attained by controlling the grain diameter or by controlling the lamellar spacing.
With respect to Ti—Al-based alloys, carbonizing and nitriding are also conducted in order to heighten the surface hardness. However, since these treatments yield carbides and nitrides, such as TiC and TiN, in the surface, there is a concern that such carbides and nitrides may cause a decrease in toughness or serve as starting points for surface fracture. In addition, the necessity of such surface treatments considerably affects the cost.
Meanwhile, it is possible in Ti—Al-based alloys to increase the hardness of the base material itself. However, the higher the hardness of the base material, the poorer the toughness thereof. Consequently, a material in which the hardness of the whole base material has been heightened cannot be used as an actual member on which high load is imposed.
Patent Document 1: JP-A-2002-356729
Patent Document 2: JP-A-2001-316743
Patent Document 3: JP-A-08-144034
Patent Document 4: JP-A-06-264203